Protector for electric circuits



July 1, 1969 J. 5. WITHERS 3,453,580

PROTECTOR FOR ELECTRIC CIRCUITS Filed March 22, 1967 W FIG. I.

Z8 0 J0 F|G.2. 92 m m United States Patent M 3,453,580 PROTECTOR FOR ELECTRIC CIRCUITS John S. Withers, Pasadena Hills, Mo., assignor to Mc- Graw-Edison Company, Elgin, 111., a corporation of Delaware Filed Mar. 22, 1967, Ser. No. 625,048 Int. Cl. H01h 85/02, 85/04, 85/46 US. Cl. 337-221 12 Claims ABSTRACT OF THE DISCLOSURE A dual-element electric fuse has a shunt, of high melting point material, connected in parallel with the series-connected fusible element and thermal cutout thereof; and, after that fusible element or that thermal cutout has responded to an overload to open, that shunt will momentarily carry that overload and then open to protect the circuit-thereby minimizing any arcing at the instant that fusible element or that thermal cutout opens.

This invention relates to improvements in protectors for electric circuits. More particularly, this invention relates to improvements in dual-element electric fuses.

It is, therefore, an object of the present invention to provide an improved dual-element electric fuse.

A dual-element electric fuse customarily has at least one fusible element and at least one thermal cutout connected in series between the terminals thereof; and that thermal cutout is intended to respond to long-continued, relatively light overloads to open, while that fusible element is intended to respond to heavy overloads or to short circuits to open. As that thermal cutout opens, or as that fusible element opens, arcing can occur; and, in certain ratings of electric fuses and under certain overloads, that arcing can be severe. That arcing can be made less severe by connecting a shunt in parallel with the series-connected heat-generating element and thermal cutout of the fuse, as shown by A. I. Fister Patent No. 3,122,619 which was granted Feb. 25, 1964. While the shunt disclosed in the said patent reduces the arcing which occurs as the thermal cutout of that fuse opens, and while that fuse is both efficient and safe in operation, some overloads can cause that fuse to emit small quantities of smoke or gas when it operates to open the circuit. It would be desirable to provide a dual-element fuse which could minimize the arcing at the thermal cutout or the fusible element thereof, which would be both efficient and safe, and which would emit substantially no smoke or gas as it operated to open the circuit. The present invention provides such a dual element fuse; and it does so by making the shunt for that fuse of a material which has a high melting point and by enclosing that shunt within a casing that contains arc-quenching material. The fact that the shunt is made from a material that has a high melting point is important; because it helps make certain that the shunt will continue to carry the overload until after the thermal cutout or the fusible element has fully opened. The fact that the shunt is enclosed within a casing that contains arc-quenching material is important; because it keeps the gases and vapors that form as the thermal cutout or the fusible element opens from contacting the shunt and the arc-quenching material surrounding that shunt, and because it keeps the gases and vapors that form as the shunt subsequently opens from intermingling with the gases and vapors that were formed as the thermal cutout or the fusible element opened. It is, therefore, an object of the present invention to provide a dual-element fuse with a shunt that is made from 3,453,580 Patented July 1, 1969 a material which has a high melting point and that is enclosed within a casing that contains arc-quenching material.

The material used in making the shunt of the dualelement fuse of the present invention has a high thermal coefiicient of resistance; and that high thermal coefficient of resistance enables that shunt to respond to overload-induced increases in the temperature thereof to increase its resistance. The increased resistance of the shunt is important, 'because it will progressively decrease the ratio of the amount of current flowing through that shunt to the amount of current flowing through the series-connected thermal cutout and fusible element prior to the opening of that thermal cutout or fusible element, and 'will thus help make certain that the shunt will continue to carry the overload until after the thermal cutout or the fusible element has fully opened. It is, therefore, an object of the present invention to provide a dual-element fuse with a shunt that is made from a material which has a high thermal coefiicient of resistance.

The shunt of the dual-element fuse provided by the present invention has weak spots; and those Weak spots fix the points where that shunt will open the circuit. Those points will be wholly enclosed within the casing for the shunt, and they will be wholly surrounded by arc-quenching material. As a result, any arcs which form as the shunt opens the circuit will be confined and will quickly be quenched. The weak spots are additionally important because they permit the shunt to be made much larger in cross section than a shunt Wire of uniform cross section, and yet limit the amount of current which normally flows through that shunt to a level as small as that which would .flow through such a shunt wire. The weak spots also are important in spreading the arcing energy, which develops as that shunt opens, over a plurality of spaced-apart arcs; and thus minimizes the clearing time of those arcs, and also reduces the amount of metal that is vaporized. It is, therefore, an object of the present invention to provide a dual element fuse with a shunt which has at least one weak spot therein, with a casing that wholly encloses that weak spot, and with arc-quenching filler in that casing which wholly surrounds that weak spot.

Other and further objects and advantages of the present invention should become apparent from an examination of the drawing and accompanying description.

In the drawing and accompanying description a preferred embodiment of the present invention is shown and described but it is to be understood that the drawing and accompanying description are for the purpose of illustration only and do not limit the invention and that the invention will be defined by the appended claims.

In the drawing, FIG. 1 is a longitudinal section through one preferred embodiment of dual-element electric fuse that is made in accordance with the principles and teachings of the present invention,

FIG. 2. is a further longitudinal section through the dual-element fuse of FIG. 1, but it shows that fuse after it has been rotated ninety degrees about its long axis,

FIG. 3 is a plan view of the combination heat-gem erating and heat-absorbing element used in the dual-element fuse of FIGS. 1 and 2,

FIG. 4 is a view, on a larger scale, of one of the fusible elements of the dual-element fuse of FIGS. 1 and 2 before that fusible element is bent,

FIG. 5 is a sectional View, on the scale of FIG. 4, through the dual-element fuse of FIGS. 1 and 2, and it is taken along the plane indicated by the line 5-5 in FIG. 1,

ice

FIG. 6 is a sectional view, on the scale of FIG. 4, through the shunt and shunt casing of the dual-element fuse of FIGS. 1 and 2, and

FIG. 7 is a side elevational view on a still larger scale of the movable, element of the dual-element fuse of FIGS. 1 and 2.

Referring to the drawing in detail, the numeral 20 denotes the housing for one preferred embodiment of dual-element electric fuse that is made in accordance with the principles and teachings of the present invention; and that housing is made from insulating material. A circular disc 22 of insulating material is dimensioned to telescope within the housing 20; and a second circular disc 24 of insulating material also is dimensioned to telescope within that housing. The disc 24 has two spaced openings through it, and those openings accommodate the legs 28 and 30 of a combination heat-generating and heat-absorbing element 26 of highly-conductive metal.

The legs 28 and 30 constitute the heat-generating por tions of the element 26; and the Wide portion of that ele ment, which is disposed to the left of the disc 24, constitutes the heat-absorbing portion of that element. The heat-generating and heat-absorbing element 26 has a tongue 32, as shown particularly by FIG. 3; and that tongue extends into an opening in the disc 22. The openings in the disc 24 which accommodate the legs 28 and 30 of the heat-generating and heat-absorbing element 26 and the opening in the disc 22 which accommodates the tongue 32 of that element hold that element away from, but fixed relative to, the interior of the housing 20. The leg 28 has an opening 34 therein, the leg 30 has an opening 36 therein, and the heat-absorbing portion of the element 26 has a small opening 38 therein.

The numeral 40 denotes a blade-type, metal terminal which has openings therein that can be set in register with the openings 34 and 36, respectively, in the legs 28 and 30 of the heat-generating and heat-absorbing element 26. Rivets 42 extend through the openings in the terminal 40 and through the openings 34 and 36 in the legs of the heat-generating and heat-absorbing element 26 to fixedly secure that terminal to that heat-generating and heat-absorbing element. In addition, some solder is applied to that terminal and to those legs to make certain that a low-resistance, electrical bond is provided between that terminal and that heat-generating and heatabsorbing element.

An opening in the terminal 40 accommodates a pin 44; and that pin has a length which is just slightly less than the outer diameter of the housing 20. As a result, the ends of that pin can abut the right-hand end face of the housing 20 and thereby limit movement of the terminal 40, and hence of the heat-generating and heatabsorbing element 26, to the left relative to that housing. A ferrule-like closure 46 has a slot adjacent the center thereof to accommodate the outer end of the terminal 40; and that closure is telescoped over the pin 44 and over the right-hand end of the housing 20. Screws 48 extend through openings in the cylindrical portion of the ferrule-like closure 46 and seat in the wall of the housing 20.

The numeral 50 generally denotes a metal fusible element; and that fusible element has a tongue 52, a narrow portion 53, a tapered portion 55, a plurality of longitudinally-extending spaced legs 54, 56, 58, 60 and 62, and a connecting portion 63. Each of the legs 54, 56, 58, 60 and 62, has a number of weak spots formed therein; and each of those weak spots is formed by two notches and an intermediate opening. An opening 64 is formed in the connecting portion of the fusible element 50. That fusible element has a width which is greater than the inner diameter of the housing 20; and hence that fusib l e element is folded to enable it to fit within that housing without touching the interior of that housing. The numeral 65 denotes a second metal fusible element; and the developed configuration of that second fusible element can be identical to the developed configuration of the fusible element 50. However, the fusible element 65 will be folded so it is the mirror image of the fusible element 50.

The numeral 66 denotes a metal terminal which has an opening that can be set in register with the opening 64 in the connecting portion of the fusible element 50;

and that terminal has a second opening that can be set in register with the corresponding opening in the connecting portion of the fusible element 65. A rivet 70 extends through the first opening in that terminal and through the opening 64 in the connecting portion 63 of the fusible element 50 to fixedly secure that fusible element to that terminal; and a rivet 71 extends through the second opening in that terminal and through the opening in the connecting portion of the fusible element 65 to fixedly secure that fusible element to that terminal. Some solder will be applied to the terminal 66 and to the riveted areas of the connecting portions of the fusible elements 50 and 65 to provide low-resistance electrical bonds between that terminal and those fusible elements. A pin 68 is pressed into the transversely-extending portion of a T-shaped opening in the terminal 66; and that pin has a length which is just slightly less than the outer diameter of the housing 20. As a result, the ends of that pin can abut the left-hand end face of the housing 20 and thereby limit movement of the terminal 66, and hence of the fusible elements 50 and 65, to the right relative to that housing. A ferrule-like closure 72 has a slot adjacent the center thereof to accommodate the outer end of the terminal 66; and that closure is telescoped over the pin 68 and over the left-hand end of the housing 20. Screws 74 extend through openings in the cylindrical portion of the ferrule-like closure 72 and seat in the Wall of the housing 20.

The numeral 76 denotes a casing for a shunt used in the dual-element fuse of FIGS. 1 and 2; and that casing can be made of any of the insulating materials customarily used to enclose electric fuses. That shunt has a copper lead 78 which extends into the opening 38 in the heat-absorbing portion of the combination heat-generating and heat-absorbing element 26. That lead is staked, and is then soldered, to that heat-absorbing portion. The other end of that lead extends through an opening in the disc 22 and through an opening in a closure '73 for the right-hand end of casing 76, and is then soldered to the right-hand end of a fusible element 80. That fusible element has weak spots 82 and 84 therein; and those weak spots are disposed wholly within the casing 76. The left-hand end of that fusible element extends outwardly through an opening in a closure in the lefthand end of the casing 76 and extends into the righthand end of the T-shaped opening in the terminal 66 which accommodates the pin 68; and that left-hand end of that fusible element is soldered within that opening. That fusible element is made from a material, such as nickel, which has a high melting point and which has a high thermal coefficient of resistance. The left-hand end of the lead 78 is soldered to the closure 73; and the resulting solder joint will fill and seal the opening in that closure. The left-hand end of the fusible element will substantialy fill the opening in the closure 75; but that left-hand end will not be soldered to that closure. As a result, relative movement between that fusible element and the casing 76, due to differences in thermal coefficients of expansion, will be permitted. Arc-extinguishing filler is disposed Within the casing 76, and that filler surrounds and intimately engages the fusible element 80.

The numeral 92 generally denotes a movable element which abuts the heat-generating and heat-absorbing element 26, and which also abuts that portion of the tongue 52 of the fusible element 50 which extends through the disc 22. That movable element has a U-shaped metal part 94, a second U-shaped metal part 96 of greater thickness,

a solid metal block 98, and one end of a helical extension spring 100. As shown particularly by FIG. 7, the upper arm of the U-shaped part 96 extends into the U-shaped part 94, and the lower arm of the U-shaped part 94, the left-hand end of the spring 100 and the block 98 are disposed within the U-shaped part 96. The latter U-shaped part is squeezed into intimate engagement with the block 98 and with the former U-shaped part to hold the various parts of that movable element in the assembled relation shown by FIG. 7. The space between the upper arm of the U-shaped part 96 and the upper arm of the U-shaped part 94 is large enough to freely accommodate that portion of the tongue 52 of the fusible element 50 which extends through the disc 22. Low melting-point solder 99 is used to mechanically-attach and electricallybond the movable element 92 to the projecting portion of the tongue 52 of fusible element 50, and also to assure low resistance connections between the various parts of that movable element. The other end of the helical extension spring 100 is held by a hook 102 that is fixedly secured to the disc 24. Another and substantially-identical movable element 104, shown by dotted lines in FIG. 2, is disposed behind and is hidden by the movable element 92 in FIG. 1. The low melting-point solder 99 also releasably secures the movable element 104 to the projecting portion of the tongue of the fusible element 65 and to the heat-generating and heat-absorbing element 26. Another and substantially-identical helical extension spring 106, shown by dotted lines in FIG. 2, and another and substantially-identical hook 108, shown by dotted lines in FIG. 2, are disposed behind and hidden by the spring 100 and the hook 102 in FIG. 1.

Arc-extinguishing filler 110 will fill the portion of the housing 20 between the disc 22 and the ferrule-like closure 72. That filler will surround and intimately engage the fusible elements 50 and 65. Further arc-extinguishing filler 112 will fill the portion of the housing 20 between the disc 24 and the ferrule-like closure 46. That filler will surround and intimately engage the legs 28 and 30 of the heat-generating and heat-absorbing element 26. The portion of the housing 20 between the discs 22 and 24 will be devoid of arc-extinguishing filler.

The electrical resistance of the fusible element 50 will be very much smaller than the electrical resistance of the fusible element 80 of the shunt; and, similarly, the electrical resistance of the fusible element 65 will be very much smaller than the electrical resistance of the fusible element 80. As a result, only a small portion of the current flowing through the dual-element electric fuse of FIG. 1 will flow through that shunt. The current which flows through that electric fuse will cause the legs 28 and 30 of the heat-generating and heat-absorbing element 26 to generate appreciable amounts of heat, and will also cause the fusible elements 50 and 65 to generate appreciable amounts of heat. The heat generated by the legs 28 and 30 will primarily flow to the terminal 40, because that terminal will be held by a fuse clip or other metal connector which will be cooler than the heat-absorbing portion of the heat-generating and heat-absorbing element 26. The current flowing through the fusible elements 50 and 65 will cause substantial amounts of heat to be generated at the weak spots of those fusible elements; but, under normal load conditions, substantial portions of those amounts of heat will flow to the terminal 66, because that terminal will be held by a fuse clip or other metal connector which will be cooler than the heatabsorbing portion of the heat-generating and heat-absorbing element 26. However, appreciable quantities of heat will flow from the legs 28 and 30 to the heat-absorbing portion of the heat-generating and heat-absorbing element 26; and appreciable quantities of heat will flow from the fusible elements 50 and 65 to that heat-absorbing portion. As long as the level of current flowing through the dual-element fuse of FIG. 1 is at or below the rated current of that fuse, the heat-absorbing portion of the heatgenerating and heat-absorbing element 26 will absorb enough heat from the low melting-point solder 99, which bonds the movable elements 92 and 104 to that heatabsorbing portion, to keep that solder from softening. As a result, as long as the rated current of the dual-element fuse of FIG. 1 is not exceeded, the movable elements 92 and 104 will remain in position to complete the electric circuit through that dual-element fuse. The shunt will respond to the current flowing through it to generate heat; but some portions of that heat will flow to the terminal 66, and other portions of that heat will flow to the heatabsorbing portion of the heat-generating and heat-absorbing element 26.

The legs 28 and 30 of the heat-generating and heatabsorbing element 26, the fusible elements 50 and 65, and the shunt will respond to overloads applied to the circuit, protected by the dual element fuse of FIG. 1, to generate more heat, and to experience increases in the temperatures thereof, Those increases in temperature will increase the resistances of that heat-generating and heat-absorbing element, of those fusible elements, and of that shunt; but, because of the high thermal coeflicient of resistance of the fusible element 80, the resistance of the shunt will increase more rapidly than will the resistances of that heat-generating and heat-absorbing element and of those fusible elements. This is important, because it will further decrease the proportion of current flowing through that shunt, and will thus make certain that the fusible element will remain intact until after the fusible elements 50 and 65 have fully opened or the movable elements 92 and 104 have fully opened.

In the event a light, but potentially hurtful, overload is applied to the circuit, protected by the dual-element fuse of FIG. 1, and in the event that overload is continued for an undue length of time, the low melting-point solder 99 which connects the movable elements 92 and 104, respectively, between the fusible elements 50 and 65 and the heat-generating and heat-absorbing element 26 will heat to its softening point; and, thereupon, the helical extension springs 100 and 106 will pull those movable elements away from the tongues of the fusible elements 50 and 65. That light, but potentially hurtful, overload caused the legs 28 and 30 of the heat-generating and heatabsorbing element 26 to generate increased amounts of heat, and also caused the fusible elements 50 and 65 to generate increased amounts of heat; and those increased amounts of heat caused the low melting-point solder 99 to soften and release the movable elements 92 and 104. In addition, that light, but potentially hurtful, overload caused the fusible element 80 of the shunt to generate increased amounts of heat. Because the thermal coefiicient of resistance of the fusible element 80 is greater than the thermal coeflicients of resistance of the heat-generating and heat-absorbing element 26 and of the fusible elements 50 and 65, the resistance of the fusible element 80 increased more rapidly than did the resistances of the heatgenerating and heat-absorbing element 26 and of the fusible elements 50 and 65. That more rapid increase in resistance caused proportionally less current to flow through the fusible element 80 and caused proportionally more current to flow through the fusible elements 50 and 65. This is important, because it made certain that the fusible element 80 of the shunt remained cool enough so it could not fuse as long as the movable members 92 and 104 were in their current-conducting positions and the fusible elements 50 and 65 were intact.

As the springs 100 and 106, respectively, move the movable elements 92 and 104 away from the tongues on the ends of the fusible elements 50 and 65, arcs may form between those tongues and the heat-absorbing portion of the heat-generating and heat-absorbing element 26. The fusible element 80 of the shunt will constitute a conductor in parallel with those arcs, and will thus limit the amount of electrical energy which can tend to sustain those arcs. If those arcs should be sustained long 7 enough to enable them to burn into the narrow portions of the fusible elements 50 and 65, the arcextinguishing filler 110, in the portion of the housing 20 between the disc 22 and the ferrule-like closure 72, will promptly extinguish those arcs. The weak spots 82 and 84 of the fusible element 80 of the shunt are incapable, by themselves, of carrying the rated current of the dualelement fuse of FIG. 1; and hence are wholly incapable, by themselves, of carrying the light, but potentially hurtful, overload which caused the low melting-point solder 99 to release the movable elements 92 and 104. As a result, within a fraction of a second after those movable elements have opened, by moving away from the projecting portions of the fusible elements 50 and 65, one or both of the weak spots 82 and 84 of the fusible element 80 of the shunt will fuse and finally interrupt the circuit between the terminals 40 and 66 of the dualelement fuse.

The weak spots 82 and 84 of the fusible element 80 of the shunt are disposed wholly within the enclosure 76, and they are intimately engaged and surrounded by the arc-extinguishing filler 90. That arc-extinguishing filler will quickly extinguish the are that will form as either or both of the weak spots 82 and 84 open.

The normal electrical resistance of the fusible element 80 of the shunt is greater than the combined normal resistances of the fusible elements 50 and 65 and of the movable elements 92 and 104; and the electrical resistance of that fusible element becomes even higher relative to the combined electrical resistances of those fusible elements and movable elements whenever an overload additionally heats the various electrically-conducing components of the dual-element fuse of FIG. 1. As as result, the fusible element of the shunt will remain relatively cool until the movable elements 92 and 104 move away from the tongues on the fusible elements 50 and 65; and then either or both of the Weak spots of that fusible element will fuse.

In the event a heavy overload or a short circuit is applied to the circuit protected by the dual-element fuse of FIG. 1, the weak spots" in one or more of the legs of the fusible elements 50 and 65 will open; and, almost immediately, at least two of the Weak spots in each and every one of the other legs of those fusible elements will open. The arcs that form as those weak spots fuse will be paralleled by the un-fused fusible element 80 of the shunt; and hence the amount of electrical energy which could tend to sustain those arcs will be limited. Consequently, the arc-extinguishing filler 110 within the housing 20 will be able to quickly extinguish those arcs.

As indicated hereinbefore, the normal electrical resistance of the fusible element 80 of the shunt is greater than the combined normal restistances of the fusible'elements 60 and 65 and of the movable elements 92 and 104; and the electrical resistance of that fusible element becomes even higher relative to the combined electrical resistances of those fusible elements and movable elements whenever an overload additionally heats the various electrically-conducting components of the dualelement fuse of FIG. 1. As a result, the fusible element of the shunt will remain relatively cool until the fusible elements 50 and 65 open; and then either or both of the weak spots of that fusible element will fuse.

Because the fusible element 80 of the shunt is made from a metal or alloy which has a melting point higher than the melting point of the metal or alloy from which the fusible elements 50 and 65 are made, that shunt can remain un-fused until after those fusible elements open or the movable elements 92 and 104 open. This is very important in reducing the intensity and duration of the arcs which form as those fusible elements open or the movable elements 92 and 104 open. Because the fusible element 80 of the shunt is made from a metal or alloy which has a thermal coefiicient of resistance greater than the thermal coefficient of resistance of the metal or alloy from which the fusible elements 50 and 65 are made, that shunt can respond to overloads to decrease the proportion of current which flows through it; and that decrease is important because it enables that shunt to remain relatively cool until the fusible elements 50 and 65 open or the movable elements 92 and 104 open.

It will be noted that the length of the fusible element is materially longer than one-half of the overall length of the housing 20. Such a length is important; because it permits that fusible element to be given a cross section which is large enough to enable that fusible element to remain un-fused when even very heavy short circuits are applied to the circuit protected by the dual-element fuse of FIG. 1, and yet permits that fusible element to have "an ohmic resistance which is large enough to keep more than just a small portion of the current flowing through that dual-element fuse from flowing through that fusible element.

The coper lead 78 has a much lower ohmic resistance than any other portion of the shunt; and hence that lead will generate much less heat than will any other portion of thatshunt. This is important; because it will keep that lead from fusing whenever either or both of the weak spots 82 and 84 openand thus will restrict all arcing due to opening of the shunt to areas which are wholly embedded within arc-extinguishing filler.

The combined heat-generating and heat-absorbing element 26 is desirable; because it provides a low level of resistance to heat flow from the legs 28 and 30 of that heat-generating and heat-absorbing element to the heatabsorbing portion of that heat-generating and heat-absorbing element. Further, that heat-generating and heatabsorbing element is desirable because it can be made by a single punching operation.

The dual-element fuse provided by the present invention forces the fusible elements 50 and 65 thereof or the movable elements 92 and 104 thereof to open before any part of the shunt will open. As a result, the arcing which occurs as those fusible elements or those movable elements open is of less intensity and of shorter duration than the arcing which occurs in prior shunt-equipped electric fuses.

Almost immediately after the weak spots in the legs of the fusible elements 50 and 65 open, the weak spots 82 and 84 of the fusible element 80 of the shunt will open. The arc-extinguishing filler within the casing 76 will quickly extinguish those arcs. The overall result is that the dual-element fuse of FIG. 1 can safely and quickly open the circuit, protected by it, without any visible evolution of smoke or gases.

The present invention also is applicable to dual-element fuses which have one rather than two fusible elements and which have one rather than two movable elements. In addition, the present invention is applicable to electric fuses that are of the single element rather than of the dualelement type.

Whereas the drawing and accompanying description have shown and described a preferred embodiment of the present invention, it should be apparent to those skilled in the art that various changes may be made in the form of the invention without affecting the scope thereof.

What I claim is:

1. An electric fuse which has a housing, which has terminals, which has a fusible element and a heat-absorbing element and a movable element that coact to define a low resistance, interruptable path through said electric fuse that normally carries the major proportion of the current flowing through the circuit to be protected by said electric fuse, which has a conductor that acts as a shunt and is connected in parallel with at least a portion of each of said fusible element and said heat-absorbing element and said movable element, which has a casing disposed within said housing and enclosing part of said conductor, and which has arc-extinguishing filler within said casing and surrounding said part of said conductor, said conductor having a portion of reduced cross-section therein which is intermediate the ends thereof and which is spaced a substantial distance away from said movable element and which will fuse as said conductor opens, whereby the point of opening of said conductor will be spaced a substantial distance away from said movable element, said reduced cross-section portion of said conductor being completely surrounded by said arc-extinguishing filler and being wholly enclosed by said casing, whereby any arc that forms as said reduced cross-section portion of said conductor fuses will be isolated from said fusible element and said heat-absorbing element and said movable element, said reduced cross-section portion of said conductor making the fusing of said conductor virtually independent of the amount of heat absorbed from said terminals by said circuit, said conductor being formed and dimensioned to remain un-fused until after said movable element has responded to an overload to move away from said fusible element or said fusible element has fused, said conductor thereafter opening to prevent further flow of current through said electric fuse and through said circuit.

2. An electric fuse which has a housing, which has terminals, which has a fusible element and a heat-absorbing element and a movable element that coact to define a lowresistance, interruptable path through said electric fuse that normally carries the major proportion of the current flowing through the circuit to be projected by said electric fuse, which has a conductor that acts as a shunt and is connected in parallel with at least a portion of each of said fusible element and said heat-absorbing element and said movable element, which has a casing disposed within said housing and enclosing part of said conductor, and which has arc-extinguishing filler within said casing and surrounding said part of said conductor, said conductor having a portion of reduced cross-section therein which is intermediate the ends thereof and which will fuse as said conductor opens, said reduced cross-section portion of said conductor being completely surrounded by said arc-extinguishing filler and being wholly enclosed by said casing, whereby any arc that forms as said reduced cross-section portion of said conductor fuses will be isolated from said fusible element and said heat-absorbing element and said movable element, said conductor being formed and dimensioned to remain un-fused until after said movable element has responded to an overload to move away from said fusible element or said fusible element has fused, said conductor thereafter opening to prevent further flow of current through said electric fuse and through said circuit, said conductor having a portlon thereof made from a material that has a higher melting point than the material of which said fusible element is made.

3. An electric fuse which has a housing, which has terminals, which has a fusible element and a heat-absorbing element and a movable element that coact to define a lowresistance, interruptable path through said electric fuse that normally carries the major proportion of the current flowing through the circuit to be projected by said electric fuse, which has a conductor that acts as a shunt and is connected in parallel with at least a portion of each of said fusible element and said heat-absorbing element and said movable element, which has a casing disposed within said housing and enclosing part of said conductor, and which has arc-extinguishing filler within said casing and surrounding said part of said conductor, said conductor having a portion of reduced cross-section therein which is intermediate the ends thereof and which will fuse as said conductor opens, said reduced cross-section portion of said conductor being completely surrounded by said arc-extinguishing filler and being wholly enclosed by said casing, whereby any arc that forms as said reduced cross-section portion of said conductor fuses will be iso- 10 lated from said fusible element and said heat-absorbing element and said movable element, said conductor being formed and dimensioned to remain un-fused until after said movable element has responded to an overload to move away from said fusible element or said fusible element has fused, said conductor thereafter opening to prevent further flow of current through said electric fuse and through said circuit, said conductor having a portion thereof made from a material that has a higher thermal coefiicient of resistance than the materials of which said fusible element and said heat-absorbing element are made.

4. An electric fuse which has a housing, which has terminals, which has a fusible element and a heat-absorbing element and a movable element that coact to define a low-resistance, interruptable path through said electric fuse that normally carries the major proportion of the current flowing through the circuit to be protected by said electric fuse, which has a conductor that acts as a shunt and is connected in parallel with at least a portion of each of said fusible element and said heat-absorbing element and said movable element, said conductor having a portion of reduced cross-section therein which is intermediate the ends thereof and which will fuse as said conductor opens, said conductor being formed and dimensioned to remain un-fused until after said movable element has responded to an overload to move away from said fusible element or said fusible element has fused, said conductor thereafter opening to prevent further flow of current through said electric fuse and through said circuit, said conductor having a portion thereof made from a material that has a higher melting point and that has a higher thermal coefiicient of resistance than the material of which said fusible element is made.

5. An electric fuse which has a housing, which has terminals, which has a fusible element and a heat-absorbing element and a movable element that coact to define a low-resistance, interruptable path through said electric fuse that normally carries the major proportion of the current flowing through the circuit to be protected by said electric fuse, which has a conductor that acts as a shunt and is connected in parallel with at least a por tion of each of said fusible element and said heat-absorbing element and said movable element, said conductor having a portion of reduced cross-section therein which is intermediate the ends thereof and which will fuse as said conductor opens, said conductor being formed and dimensioned to remain un-fused until after said movable element has responded to an overload to move away from said fusible element or said fusible element has fused, said conductor thereafter opening" to prevent further fiow of current through said electric fuse and .through said circuit, said conductor having a portion thereof made from a material containing nickel, and none of said fusible element and said heat-absorbing element and said movable element having nickel-containing material therein.

6. An electric fuse which has a housing, which has termi nals, which has a fusible element and a heat-absorbing normally carries the major proportion of the current flowelement and a movable element that coact to define a lowresistance, interruptable path through said electric fuse that ing through the circuit to be projected by said electric fuse, which has a conductor that acts as a shunt and is connected in parallel with at least a portion of each of said fusible element and said heat-absorbing element and said movable element, which has a casing disposed within said housing and enclosing part of said conductor, and which has arc-extinguishing filler within said casing and surrounding said part of said conductor, said conductor having a portion of reduced cross-section therein which is intermediate the ends thereof and which will fuse as said conductor opens, said reduced cross-section portion of said conductor being completely surrounded by said arc-extinguishing filler and being wholly enclosed by said casing, whereby any arc that forms as said reduced cross-section portion of said conductor fuses will be isolated from said fusible element and said heat-asborbing element and said movable element, said conductor being formed and dimensioned to remain un-fused until after said movable element has responded to an overload to move away from said fusible element or said fusible element has fused, said conductor thereafter opening to prevent further flow of current through said electric fuse and through said circuit, said low-resistance interruptable path also including a heat-generating element, and said conductor having a length greater than one-half the combined lengths of said fusible element and said heat-absorbing element and said heat-generating element.

7. An electric fuse which has a housing, which has terminals, which has a fusible element and a heat-absorbing element and a movable element that coact to define a low-resistance, interruptable path through said electric fuse that normally carries the major proportion of the current flowing through the circuit to be protected by said electric fuse, which has a conductor that acts as a shunt and is connected in parallel with at least a portion of each of said fusible element and said heat-absorbing element and said movable element, which has a'casing disposed within said housing and enclosing part of said conductor, and which has arc-extinguishing filler within said casing and surrounding said part of said conductor, said conductor having a portion intermediate the ends thereof which will fuse as said conductor opens, said portion of said conductor being completely surrounded by said arc-extinguishing filler and being wholly encased by said casing, whereby any are that forms as said portion of said conductor fuses will be isolated from said fusible element and said heat-absorbing element and said movable element, said conductor being formed and dimensioned to remain un-fused until after said movable element has responded to an overload to move away from said fusible element or said fusible element has fused, said conductor thereafter opening to prevent further flow of current through said electric fuse and through said circuit, said conductor normally carrying only a small portion of the current flowing through said electric fuse, and said conductor responding to overloads to carry proportionately less of the current flowing though said electric fuse.

8. An electric fuse which has a housing, which has terminals, which has a fusible element and a heat-absorbing element and a movable element that coact to define a low-resistance, interruptable path through said electric fuse that normally carries the major proportion of the current flowing through the circuit to be proctected by said electric fuse, which has a conductor that acts as a shunt and is connected in parallel with at least a portion of each of said fusible element and said heatabsorbing element and said movable element, said conductor having a portion intermediate the ends thereof which will fuse as said conductor opens, said portion of said conductor being made from a material that has a higher melting point and a higher thermal coefficient of resistance than the materials of which said fusible element and said heat-absorbing element are made, said conductor perceptibly decreasing the amount of current flowing through said electric fuse after said movable element moves away from said fusible element, whereby said conductor reduces the amount of energy which it must dissipate as it subsequently opens the circuit through said electric fuse.

9. An electric fuse which has a housing, which has terminals, which has a fusible element and a heat-absorbing element and a movable element that coact to define a low-resistance, interruptable path through said electric fuse that normally carries the major proportion of the current flowing through the circuit to be protected by said electric Ifuse, which has a conductor that acts as a shunt and is connected in parallel with at least a portion of each of said fusible element and aid heat-absorbing element and said movable element, said conductor having a portion intermediate the ends thereof which will fuse as said conductor opens, said portion of said conductor being made from a material that has a higher melting point and a higher thermal coefficient of resistance than the materials of which said fusible element and said heat-absorbing element are made, said low-resistance, interruptable path also including a heat-generating element, and said heat-absorbing element and said heat-generating element being formed from one piece of metal.

10. An electric fuse which has a housing, which has terminals, which has a fusible element and a heat-absorbing element and a movable element that coact to define a low-resistance, interruptable path through said electric fuse that normally carries the major proportion of the current flowing through the circuit to be protected by said electric fuse, which has a conductor that acts as a shunt and is connected in parallel with at least a portion of each of said fusible element and said heatabsorbing element and said movable element, said conductor having a portion intermediate the ends thereof which will fuse as said conductor opens, said portion I of said conductor being made from a material that has a higher melting point and a higher thermal coefficient of resistance than the materials of which said fusible element and said heat-absorbing element are made, said low-resistance interruptable path also including a heatgenerating element, and a single piece of metal constituting both said heat-absorbing element and said heat-generating element of said lowresistance, interruptable path, said heat-absorbing element being a part of said single piece of metal which has a relatively large cross-section, said heat-generating element being a part of said single piece of metal which has a relatively small cross-section.

11. An electric fuse which has a housing, which has terminals, which has a fusible element and a heat-absorbing element and a movable element that coact to define a low-resistance, interruptable path through said electric fuse that normally carries the major proportion of the current flowing through the circuit to be protected by said electric fuse, which has a conductor that acts as a shunt and is connected in parallel with at least a portion of each of said fusible element and said heatabsorbing element and said movable element, said conductor having a portion which will fuse when said movable element moves away fnom said fusible element and having a second portion which will not f-use when said movable element moves away from said fusible element, the first said portion of said conductor being made from a material that has a higher melting point and a higher thermal coefiicient of resistance than the materials of which said fusible element and said heat-absorbing element and said second portion of said conductor are made, said second portion of said conductor being made of lowresistance material and extending to, and being secured to, said heat-absorbing element, whereby said second portion of said conductor spaces the first said portion of said conductor away from said movable element and said heat-absorbing element.

12. An electric fuse which has a housing, which has terminals, which has a fusible element and a heat-absorbing element and a movable element that coact to define a low-resistance, interruptable path through said electric fuse that normally carries the major proportion of the current flowing through the circuit to be protecte/d by said electric fuse, which has a conductor that acts as a shunt and is connected in parallel with at least a portion of each of said fusible element and said heat-absorbing element and said movable element, said conductor being formed and dimensioned to remain unifused until after said movable element has responded to an overload to move away from said fusible element or said fusible element has fused, said conductor thereafter opening to prevent further flow of current through said electric fuse and through said circuit, a substantial portion of the length of said conductor being enclosed within a casing disposed within said housing, said substantial portion of the length of said conductor having a weak spot therein which will fuse as said conductor opens, said weak spot being completely surrounded by arc-extinguished filler within said casing, whereby any are that forms as said weak spot fuses will be isolated from said dusible element and said heat-absorbing element, the end of said casing which is close to said heatabsorbing element being sealed to prevent the escape of vaporized metal and flames therethrough.

14 References Cited UNITED STATES PATENTS VOLODYMYR, Y. MAYEWSKY, Primary Examiner. 10 H. B. GILSO-N, Assistan t Examiner.

U.S. Cl. X.R. 

